1. Field of the Invention
The present invention relates to a GFCI device that automatically resets when it is properly wired and power is applied to the device. According to one embodiment, the present invention includes a GFCI device with a power-on reset circuit that directs an electric current through a circuit passing through a sense transformer to electrical ground to create a magnetic flux in the transformer upon power-up. A detector within the GFCI device detects this condition and outputs a signal to reset the device when the power-on reset circuit no longer provides enough current for the detector to detect the magnetic flux. A further embodiment of the present invention includes a microprocessor programmed to automatically reset the properly wired device each time power is applied to the device.
2. Description of Related Art
GFCI devices typically trip and prevent current from flowing from the line terminals to the load terminals in response to the detection of a ground fault condition at an AC load. For example, a ground fault condition can result when a person comes into contact with the hot conductor and earth ground at the same time, thus creating an alternative current path through the person, a situation that can result in serious injury or death. The GFCI device detects this condition with a sensing transformer that detects an imbalance between the currents flowing in the hot and neutral conductors of the AC supply, as will occur when some of the current on the hot conductor is diverted to ground and not returned on the neutral conductor. In most GFCI devices, when the detected imbalance exceeds a predetermined threshold, a mechanically latched circuit breaker within the GFCI device is immediately tripped to create an open circuit condition, thereby opening the hot and/or neutral conducting paths of the AC line and removing all power from the load. Many types of GFCI devices are capable of being tripped not only due to contact between the hot side of the AC load and ground, which is known as a ground fault, but also due to grounded neutral fault, which is caused by a connection between the neutral side of the AC load and ground. This latter type of fault, which may result from a defective load or from improper wiring, is potentially dangerous because it can prevent a conventional GFCI device from tripping due to a ground fault at the desired threshold level.
GFCI devices may be connected to fuse boxes or circuit breaker panels to provide central protection for the AC wiring throughout a commercial or residential structure. More commonly, however, GFCI devices are incorporated into electrical receptacles that are designed for installation at various locations within a building. A typical GFCI receptacle includes test and reset push buttons and an incandescent lamp or light-emitting diode (LED), which indicates that the circuit is operating normally. When an actual ground fault occurs in the protected circuit, or when the test button is depressed, the GFCI device trips and an internal circuit breaker opens one or both sides of the AC line. The tripping of the internal circuit breaker causes the reset button to pop out and the LED to be extinguished, providing a visual indication that a ground fault has occurred. To reset the GFCI device, the reset button is depressed which closes and latches the circuit breaker, and which again illuminates the LED.
According to one known variety of GFCI devices, when power is initially applied, e.g., during installation, or reapplied, e.g., after a power failure/outage, to a properly wired device where AC power is connected to the line side of the device as opposed to the load side, a user must manually press the reset button to activate the GFCI device. Another type of known GFCI device will not trip when power is lost due to a power outage. According to this type of GFCI, the contacts stay connected during the outage and when power is restored to the device the GFCI device continues to provide power to the load, without the need to reset the device. A third type of GFCI device will automatically reset itself after it trips and power is present at the device.
Some previously known GFCI devices that reset automatically when power is restored to the device use a significant amount of power to implement and maintain the reset function. This is because the mechanism by which the reset condition is implemented and maintained, such as a solenoid or a relay device, requires that constant power be applied. Accordingly, there is a need for an improved GFCI device and method of operating the same where the device is automatically reset when the device is properly wired and the power required to operate and maintain the reset operation is reduced.
In addition to ground fault detection/protection, protection from miswiring is also needed. Specifically, as described above, GFCI receptacles may be erroneously connected with the incoming AC source conductors tied directly to the load or feedthrough terminals of the receptacle rather than to the line terminals. Because ground fault protection at the receptacle face terminals may not be provided under these conditions, additional measures must be taken to protect the user when such a miswire condition occurs.
To provide protection, a GFCI device must be able to remove line current from load connections when a ground fault is detected. Typically, a GFCI has two load connections or terminals. The load terminals, often in the form of screws, are connected to downstream receptacles, which are also provided with ground fault protection. Also, the face terminals of the GFCI device provide power and ground fault protection to an appliance plugged directly into the GFCI. The sensing transformer of the GFCI detects an imbalance between the current passing through the transformer on the hot conductor and the current returning on the neutral conductor. In a common configuration, the sensing transformer is placed between the line terminals and the circuit interrupter.
If a GFCI is miswired, that is, the line connections and the load connections are reversed, protection will only be supplied to downstream receptacles connected to the screw terminals. The face terminal, and any device connected thereto, will not be protected from ground faults because current must pass through the sense transformer for the imbalance in current to be detected. Accordingly, when a GFCI is miswired, current will flow from the incorrectly wired load terminals to the face terminals without passing through the sense transformer.
In one approach for addressing the above-described issues, a GFCI employs a miswire plate, which is latched before the GFCI leaves the factory. Upon proper installation and power up, current is passed directly through the solenoid without passing through the sensing transformer. The solenoid fires and the miswire plate is unlatched. However, when the miswire plate is unlatched it is no longer in the circuit and no longer provides miswire protection.
According to this approach, the miswire circuit does not subsequently test if the device is wired correctly. That is, if the GFCI device is removed from the wall and installed in a new location, the installer does not know whether the device was reinstalled correctly or miswired. If the device is miswired when re-installed, the face terminal would not have ground fault protection.
In another approach that has been considered, a GFCI is tested for proper wiring by manually pressing a test button and then manually pressing a reset button on the GFCI. If working properly, pressing the test button causes the GFCI to trip mechanically. When the reset button is pressed, it actuates a switch that provides a ground fault to the device, which causes the GFCI to fire a solenoid. If the solenoid does not fire, the reset button will not latch due to the GFCI being miswired.
One drawback to this approach is that it requires the user to manually press both the test and reset button in order to test if the GFCI has been installed correctly. Accordingly, there is a need for an improved method and device for automatically testing whether a GFCI is properly wired each time power is applied to the device and for automatically resetting the device if the device is properly wired.